As one of the methods of producing a polyester fiber having strength and elongation characteristics suitable for woven fabrics and knitted fabrics at a low cost of production, a so-called "direct spin draw system" has been proposed as described, for example, in Japanese Patent Application Kokai Publication No. 54-18918. According to this system, melt-and-spun polyester fibers are wound a plurality of times on a pair of heated godet rollers (hereinafter referred to as the "heating godet rollers") and on separate rollers to raise the fiber temperature, are again wound on another pair of heating godet rollers rotating at a higher speed and separate rollers, a plurality of times, respectively, and are stretched between both groups of rollers.
According to this production method, however, the fibers must be wound a large number of times on the heating godet rollers through the separate rollers so as to sufficiently heat the fibers. Therefore, when the number of spun yarns is increased to improve productivity, adjacent yarns are likely to overlap with one another to cause yarn breakage, the equipment becomes large in scale and power consumption of the heating rollers becomes greater.
To solve these problems, Japanese Patent Application Kokai Publication No. 62-141118 proposes a method which takes up the melt-and-spun yarns on non-heating rollers rotating at a speed of higher than 4,000 m/min, then draws the yarns while jetting a heating steam to the yarns to heat them to a temperature higher than their glass transition point. Though this production method provides the advantage that the separate rollers become unnecessary, it is not free from the following problems because the method is fundamentally based on the premise of high speed take-up at higher than 4,000 m/min. Namely, when the melt-and-spun yarns travel between a spinneret and a high speed take-up roller, a structure rich of a crystalline phase is formed due to cooling of the yarns by air and to a drawing operation, and even when drawing and heat-treatment (heat-set) are thereafter applied so as to obtain a predetermined elongation, the degree of orientation (birefringence) of the fibers finally obtained is low, so that the application of the fiber is remarkably limited.
When the take-up speed is lowered below 4,000 m/min in this method to obtain a fiber having a high degree of orientation (birefringence), it has been found out that dyeing specks occur during a dyeing step and the resulting fiber cannot be used for practical applications.